1. Field of the Invention
The present invention relates to magnetoresistive (MR) heads, and more particularly to protecting MR heads from electrostatic discharge (ESD).
2. Background Information
In magnetic data storage devices, such as those used for tape or hard disk drives (HDD), transducer heads are utilized for reading and writing data on magnetic media. Such transducer heads typically include transducer elements for reading (readers) and writing (writers) magnetic information on magnetic media. The transducer elements are typically small in size and are deposited on a wafer using lithographic techniques.
The readers include magnetoresistive (MR) sensors comprising thin film sheet resistors that are highly susceptible to damage from electrostatic discharge, (ESD), either through Joule heating from high currents or from dielectric breakdown. The writers are inductive, and alone are much less susceptible to ESD damage from high currents because they are built to sustain high writer currents. However, as shown by an example in FIG. 1A, a typical HDD or tape MR transducer head 10 comprises a piggyback structure, wherein components of each reader 12 and writer 14 are separated by one or more relatively thin insulation layers such as oxide layers 16. The reader 12 and writer 14 are encapsulated by a substrate 18 and a closure 19. The writer 14 is stacked on top of the reader 12 vertically in thin film layers. Referring now also to a schematic view of the MR transducer head 10 in FIG. 1B, a reader 12 includes shields 12A-B and a read sensor 12C disposed therebetween. A writer 14 includes writer poles 14A-B separated by a writer gap 14C. Further, leads 22 provide conductive paths to the reader 12 and the writer 14.
The insulation layers between the readers and writers are susceptible to dielectric breakdown with damaging electric field levels on the order of 1×108 to 2×108 V/m. One specific failure mode that takes place in the piggyback structured MR as in FIG. 1 is shorting between the reader 12 and writer 14 within the same transducer element 10. For an insulation layer thickness of 0.6 microns, a voltage differential of 60-120 V will result in dielectric breakdown leading to ESD failure and resulting damage.
ESD damage is a detractor for production yield during the transducer head manufacturing process. ESD damage can manifest in MR sensor resistance value as over high limit (OHL), as under low limit resistance (ULL) measurement and any value in-between. Subtle ESD damage can also be magnetic in nature and may not be readily observable as a change in resistance. The likelihood of shorting events between readers and writers due to ESD can be as high as the typical OHL failure mode. Extant tape heads contain upwards of 30 to 40 reader-writer pairs per tape head, such that a per-transducer loss as low as 0.1% translates to a large loss of 3 to 4% loss of tape heads.
Referring to FIG. 2, in tape head manufacturing, a flexible cable 20, which is made of an insulating material, such as Kapton (polyimide), is bonded to the tape head module to provide current passage to all the readers and writers via metal leads 22. The leads 22 are connected to the readers/writers and are sandwiched between insulating Kapton layers to provide proper electrical insulation between the leads for mechanical structure and to prevent electrical shorting between the leads. Similar flexible cables are also being used in HDD head manufacturing.
The Kapton cable surface may collect electrostatic charge through tribocharging due to handling or repeatedly sliding between fixtures. In this example, an electrostatic charge is shown as a negative charge 24 which induces a positive charge 25 on the surface of the metal leads 22. A uniform distribution of a negative charge 26 remains in the bulk of the metal lead 22, sustaining conservation of charge, and thus, maintaining charge neutrality. However, because Kapton is made of insulating material, tribocharging on the Kapton surface is localized and results in uneven surface charge distribution. As a result, the charge induced on the leads 22 varies with the amount of localization on the Kapton surface. This variation in charge induced on the leads 22 causes a potential difference between the leads 22. Once such potential difference exceeds the dielectric breakdown threshold of the insulation layer 16 (FIG. 1) between the readers 12 and writers 14, electrostatic discharge between the leads damages the tape head. The same occurs for HDD heads.